Prime Minister David Cameron (right) makes a joint appearance with Mayor of London Sadiq Khan (centre) as they launch the Britain Stronger in Europe guarantee card at Roehampton University in West London. Yui Mok/PA Images David Cameron and London's Mayor, Sadiq Khan, have formed an unlikely alliance in the fight to keep Britain in the European Union.

The two politicians shared a platform for the first time at the launch of a "Britain Stronger in Europe" battle bus on Monday.

A Brexit 'guarantee card' was unveiled at the event. The card will be handed out by campaigners across the country and outlines five things which will be guaranteed if Britain stays in the EU.

The pledges include:

Full access to the EU's single market — Khan says over 500,000 jobs in London rely on the EU.

The protection of workers' rights, including anti-discrimination laws and paid leave.

A special status, which would mean never joining the Euro and having control over the UK's borders.

Keeping the European Arrest Warrant.

And stability for the UK.

Speaking at the event, Khan explained: "The vote in just 24 days time is one of the most important political decisions many of us will take in our lifetime. The stakes could not be higher."

"That's why I say to young people get involved, get registered, have your say and make sure you make a decision that will affect your future and the future of generations to come."

The unexpected act of reconciliation comes just weeks after the prime minister accused Sadiq Khan of having links to Islamic extremists during the London mayoral election, which prompted Labour to accuse the Conservative party of racism.

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However, the newly-elected mayor appears to have bridged their political gap for the sake of the EU referendum. While appearing on ITV's "Peston on Sunday," he said: "Is it in London's interests for the mayor of London to be at permanent war with the Conservative prime minister?"

"We're never going to be best friends, but what is important is that the mayor of London argues the case for London and for Londoners to remain in the European Union. This debate is far more important than David Cameron or me."

During the event, both Cameron and Khan acknowledged the bad blood between them, but the prime minister lauded the campaign's "unity of purpose."

Khan isn't the first Labour politician to have shared a platform with Conservative David Cameron in a bid to keep Britain in the EU — both Harriet Harman and Tessa Jowell have spoken alongside the PM — but as arguably the most powerful labour politician in the country, he is the most important.